In the processing of semiconductor devices, particularly metal-oxide-semiconductor (MOS) devices, a silicon nitride layer is often used atop a silicon or silicon dioxide layer to protect the underlying silicon or silicon dioxide from attack during a subsequent wet chemical or oxidation steps. The silicon nitride layer can also be maintained on the device and used as a gate dielectric for the MOS device itself, either alone or with other dielectric layers such as silicon dioxide. A silicon nitride layer or a plurality of silicon dioxide and silicon nitride layers are often used as a gate dielectric, a tunnel dielectrics, or an intergate dielectric (between a floating gate and a control gate) on EPROMs or EEPROMs.
The silicon nitride layers must themselves be resistant to attack by wet chemicals including sulfuric acid, hydrogen peroxide, phosphoric acid, hydrofluoric acid and high temperature oxidants including oxygen and steam. At a minimum, the silicon nitride layer often must be attacked very slowly by the wet chemicals and high temperature oxidants. Two methods of modifying the silicon nitride etch rate include annealing and oxidizing the silicon nitride.
A silicon nitride layer may be annealed to lower its etch rate. However, a heat treatment may not lower the etch rate sufficiently to avoid the need for a relatively thicker silicon nitride layer to be certain that sufficient silicon nitride protection remains during and after each subsequent etching or oxidation step. However, a relatively thicker silicon nitride layer is undesirable because it inhibits the precise ability to define the underlying patterns that thinner layers can achieve. Moreover, the relatively thicker layer may degrade device performance when used as a gate dielectric, tunnel dielectric, or intergate dielectric.
Silicon nitride typically is converted to silicon dioxide during a high temperature steam oxidation, such as a field oxidation step. The amount of silicon nitride converted to silicon dioxide depends on the time and temperature of the steam cycle. A similar oxidation of silicon nitride occurs with sulfuric acid because sulfuric acid acts as an oxidizing agent. Sulfuric acid is commonly used for photoresist stripping or as part of a furnace preclean sequence. Although silicon dioxide is more resistant to attack, for example, by phosphoric acid, silicon dioxide is severely attacked by a hydrofluoric acid solution or other wet chemical oxide etching solutions that are typically used in many semiconductor process sequences. The repetitive oxidations of a silicon nitride layer and exposures to wet chemicals may cause the silicon nitride layer to become too thin and fail as as a protective layer.